This invention relates to apparatus for sampling aerosols in a gas flow and, more particularly, to apparatus adapted for isokinetic sampling of a gas flow.
It is difficult to sample a fluid without disturbing the condition or composition of a fluid flow system. Any disturbance may have adverse effects on the system composition and prevent the extraction of a truly representative sample of the fluid. In particular, accurate sampling of aerosols, which winds greater than 2m/sec, etc.), is particularly difficult. aerosol may be defined as a group of solid particles or liquid particles suspended in a gaseous medium. The size range of these particles is generally between 10 nanometers and 100,000 nanometers in diameter. In an aerosol, the large particles account for most of the mass or weight of an aerosol. From observation, it appears that the particle sizes between 100 nanometers and 1,000 nanometers cause the greatest health impairment and also cause the greatest decrease of visibility in the atmosphere.
Sampling apparatus heretofore available for measuring particulate matter under isokinetic flow conditions have required considerable manual adjustment during use. An apparatus of this type is shown in Boubel, U.S. Pat. No. 3,841,145. This sampler comprises a generally cylindrical tube and includes an inlet nozzle disposed at the forward end of the tube and adapted for insertion into a conduit through which a gas is flowing. A pitot tube is attached to the nozzle and is adapted for insertion into the conduit with the nozzle for measuring the velocity of gas flowing through the conduit. A filter is disposed in the tube downstream of the nozzle for collecting particulate matter. The filter is disposed in a housing which comprises upstream and downstream sections removably connected together to facilitate rinsing of the sampler upstream of the filter as is necessary fully to account for particulate matter removed from the flow.
Another sampling apparatus is shown in Logan, U.S. Pat. No. 3,921,458. This system consists of a circularly arcuate tube having a first end to be disposed outside a conduit through which fluid travels along a flow path, and a second end disposed inside the conduit at or near its center in a plane aligned to the flow path of the fluid and substantially parallel thereto. The second end of the tube faces the path of fluid flow for accurate sampling of the fluid with minimal disturbance to flow. The first end of the tube is connectable to sampling means capable of withdrawing fluid from the conduit at the same linear velocity of the fluid flowing through the conduit. A supporting sleeve is attached to the conduit by an external weld joint where the tube enters the conduit and acts in cooperation with a combined seal to seal the probe against leakage, control the extension of the probe into the conduit as well as align it with the axial path of flow of fluid through the conduit for accurate sampling of the fluid composition.
A further sampling apparatus is shown in Boubel et al., U.S. Pat. No. 3,965,748. In this system, a sampler for collecting particulate samplings in gaseous emissions includes means for automatically matching the volume rate of flow through the sampler to the flow in a stack. Pressure drops and temperatures in the stack and in the sampler are continuously detected and applied to calculating circuitry which controls a valve in the sampler for maintaining isokinetic flow conditions. A filter is removably disposed in the sampler to collect particulate material from the gaseous emission during the isokinetic flow which is obtained. Flow and flow rate are also detected and displayed.